During lithographic manufacturing processes masks are illuminated by ultra violet radiation in order to imprint patterns onto at least partially transparent objects. Some mask inspection tools inspect masks by using visible light. The resolution of each of the lithographic and inspection processes is inversely proportional to the wavelength of radiation using during the respective process. Accordingly, small defects (in relation to the wavelength of visible light) can be imprinted during the lithographic process while remain undetected during the mask inspection process.
For example, if small pinholes (lack of opaque material) are formed in the mask, transmissive bright field light rays that pass through the small pinholes are scattered thus causing only a non-noticeable fraction of transmissive bright field light rays to be detected by a detector. In addition, transmissive bright field illumination enhances various phenomena that are not regarded as defects (or regarded as acceptable defects that should be ignored of). These phenomena can be represented as transmissive dark spots (group of pixels that have a high gray level) on an image of the mask and can cause many false alarms. These phenomena can include background scattering centers such as dust, air bubbles, and contamination.
Accordingly, an inspection process can miss small and sub-pixel size defects, like pinholes and scratches that should be detected. In addition, pure transmissive brightfield illumination undesirably enhances various background scattering centers such as dust, air bubbles, contaminations thus causing false alarms.
FIG. 1a illustrates the problem of small pinholes invisibility. Large (diameter d0>>λ) and small (diameter d1˜λ) pinholes 2 and 2′ being illuminated by uniform transmissive bright field I0 (on-axis quasi plane wave) transmit strongly modulated signals I1 (d0) 3 and I1 (d1) 3′ correspondingly. The small pinhole signal I1 (d1) 3′ is significantly weaker than the large one I1 (d0) 3 due to diffraction effect. The additional weakening of small pinholes signal relative to those of large is provided by the diffractive limits of imaging optical system, at least partially transparent objective lens and CCD array 7. As the results the large and small pinholes gray level ratio I3 (d0)/I3 (d1) (as detected by CCD array 7) is too high (˜103˜104) to be simultaneously visible within the same dynamic range. I3(d0) is denoted 5 while I3(d1) is denoted 5′. I2(d0) 4 represents the large pinhole signal that impinges onto CCD array 7 while I2(d1) 4′ represents the small pinhole signal that impinges onto CCD array 7. It is noted that at least partially transparent objective lens 6 is positioned between the at least partially transparent object 1 (that includes pinholes 2 and 2′) and CCD array 7.
The relationships between the intensities of the mentioned above signals can be calculated by the following equations:
                    I        1            ⁡              (                  d          ⁢                                          ⁢          0                )                            I        1            ⁡              (                  d          ⁢                                          ⁢          1                )              ∝            [                                                  J              1                        ⁡                          (                                                d                  ⁢                                                                          ⁢                  0                                λ                            )                                                          J              1                        ⁡                          (                                                d                  ⁢                                                                          ⁢                  1                                λ                            )                                      ·                              d            ⁢                                                  ⁢            1                                d            ⁢                                                  ⁢            0                              ]        2  where J1(x) is a Bessel function of the first kind of order unity and MTF are the transfer functions of the CCD and of the lens
                              I          2                ⁡                  (                      d            ⁢                                                  ⁢            0                    )                                      I          2                ⁡                  (                      d            ⁢                                                  ⁢            1                    )                      ≈                                        I            1                    ⁡                      (                          d              ⁢                                                          ⁢              0                        )                                                I            1                    ⁡                      (                          d              ⁢                                                          ⁢              1                        )                              ·                                    MTF            Lens                    ⁡                      (                          d              ⁢                                                          ⁢              0                        )                                                MTF            Lens                    ⁡                      (                          d              ⁢                                                          ⁢              1                        )                                                            I          3                ⁡                  (                      d            ⁢                                                  ⁢            0                    )                                      I          3                ⁡                  (                      d            ⁢                                                  ⁢            1                    )                      ≈                                        I            2                    ⁡                      (                          d              ⁢                                                          ⁢              0                        )                                                I            2                    ⁡                      (                          d              ⁢                                                          ⁢              1                        )                              ·                                    MTF            CCD                    ⁡                      (                                          d                ⁢                                                                  ⁢                                  0                  ·                  m                                            pixel_size                        )                                                MTF            CCD                    ⁡                      (                                          d                ⁢                                                                  ⁢                                  1                  ·                  m                                            pixel_size                        )                              
There is a growing need to provide an effective method and system for defect detection.